An elevator installation is installed in a building and substantially consists of a car which is carried by one or more carrier elements. In a known elevator installation a drive acts on the carrier elements in order to displace the car along substantially vertical guide rails. The elevator installation is used to convey people and goods over individual or a number of floors within the building.
The carrier elements can be embodied as individual round ropes made of metal (steel). By way of example, each round rope has a diameter of approximately 8-10 mm and is stranded from individual strands, which in turn consist of individual wires. The round ropes are usually not sheathed, and so the stranding is visible on the surface thereof. Depending on the application, it is also possible for these carrier elements to be sheathed; then the individual strands or wires are not visible.
However, the carrier elements can also be flat ropes or flat belts, which have a rectangular cross section; i.e., such flat belts are wider than high (thick). A flat belt consists of individual thin steel ropes or tension members, which are embedded in a plastic and are sheathed by the latter. The steel ropes, or tension members, take up tensile forces while the plastic, inter alia, protects the steel ropes from external environmental effects and, for example, ensures a desired traction on a drive pulley of the elevator installation.
The carrier elements can also consist of tension members in the form of non-metallic ropes and strands. Such non-metallic strands, or tension members, can for example be made of carbon or silicon fibers, of aramid or glass fibers, etc. These non-metallic ropes or strands are generally embedded in a plastic sheath. The ropes or strands take up tensile forces while the plastic sheath, inter alia, protects the ropes or strands from external environmental effects and, once again, ensures a desired traction on a drive pulley of the elevator installation. These non-metallic carrier elements can likewise be embodied with a round design in the form of flat ropes or as flat belts.
In order to ensure the safety of elevator installations, the utilized carrier elements are tested at regular intervals. In the process, a test is undertaken to see whether defects, such as kinks, loop formation, breaks of strands and wires, loosenings of the outer layer, or pinches have occurred. Use can be made of various technologies and methods for testing. By way of example, known methods are based on a visual inspection by a servicing engineer or a measurement of electrical (e.g. resistance) or magnetic (e.g. magnetic flux) properties.
In order to test the carrier elements having metallic ropes or strands, use can for example be made of methods in which the carrier element is exposed to magnetic fields and changes in the magnetic flux are determined. U.S. Pat. No. 5,198,765 has disclosed a method in which a magnetic field is generated by means of a magnetizing head, with a carrier element being moved through said field in the axial direction. In the process, the carrier element is magnetically saturated at a first location. Provision is made at a further location for a scanning apparatus, by means of which magnetic flux changes in the carrier element are determined, with said magnetic flux changes being related to a cross-sectional change in the carrier element. U.S. Pat. No. 5,804,964 describes that leakage flux can also occur if individual wires are interrupted and that this leakage flux emerges from the carrier element and is detected by means of a Hall sensor.
In order to test carrier elements with non-metallic carrier elements, solutions with so-called indicator strands are known, with the latter being inserted into the carrier element. Wear and tear of the carrier elements can be identified by means of these indicator strands.
The inspection of the carrier element becomes more difficult if it has a sheath. Damages which are already so serious that they are already visible from the outside can be identified despite the sheath. However, the sheath prevents emerging damage, which is initially still small, from being visible from the outside. This externally non-visible damage to the tension member constitutes a potential risk to safety. A purely visual inspection by a servicing engineer therefore does not suffice.